The Mechanism of the Cleavage of Phenyl-t-butylcarbinol by Chromic Acid

Abstract

Phenyl-t-butylcarbinol is oxidized by chromic acid in 86.5% acetic acid solution to phenyl t-butyl ketone and cleaved to t-butyl alcohol and benzaldehyde. The yields of cleavage products approach 60-70% in dilute solutions of chromic acid containing sodium acetate, but fall to a low value in concentrated solutions of chromic acid or in dilute solutions of chromic acid in the presence of manganous or cerous salts. The corresponding deuterated carbinol, l-deutero-l-phenyl-2,2-dimethylpropanol-1, is oxidized to ketone, t-butyl alcohol and deuterobenzaldehyde; the yield of cleavage products approaches the =me maximum limit but is generally higher than with the ordinary carbinol. The rate of oxidation and cleavage for the deuterated carbinol is only about 1/10 that for the ordinary carbinol. These facts, together with those from earlier investigations, suggest that the carbinol is oxidized directly to ketone in the rate controlling step of the over-all reaction. The cleavage is caused by an unstable intermediate containing either tetravalent or pentavalent chromium, which is formed as a result of the primary oxidation step. A mechanism is postulated to account for the kinetics, the cleavage yields and isotope effects.Mosher and Whitmore2 discovered that the chromic acid oxidation of methyl-t-amylcarbinol yields t-amyl alcohol in addition to the expected ketone. Additional examples of this cleavage reaction have been discovered and investigated by Mosher and his collaborator^.^ The present research was undertaken to elucidate the mechanism of the cleavage process, and to correlate it with the mechanism of the chromic acid oxidation of isopropyl alcoh01.~ Chromic acid in 86.5% acetic acid has now been shown to oxidize and cleave phenyl-t-butylcarbinol and l-deutero-l-phenyl-2,2-dimethylpropanol-l according to equations 1 and 2.after standing for a month at room temperature to 42-44.5'. A sample of the carbinol was converted to its acid phthalate,6 m.p. 142-143", and then regenerated; it melted at 44-45". Another sample, prepared by the lithium aluminum hydride reduction of pivalophenone (see the preparation of the deuterocarbinol below) also melted at 44-45'.l-Deutero-l-phenyl-2,2-dimethylpropanol-1 .-Pivalophenone7 was converted to its oxime,8 m.p. 165-166", and then regenerated by refluxing the oxime for 8 hours with N hydrochloric acid. (13 mm.). A solution of 9.4 g. of this ketone in 100 cc. of anhydrous ether was added to a solution of 1.2 g. of LiAIDa (Metal Hydrides Co; 100% isotopic purity) in 200 cc. of anhydrous ether. The resultant slurry was decomposed with wet ether, with water and then with dilute sulfuric acid. The ether solution was washed with aqueous bicarbonate solution, with water, and then dried with anhydrous sodium sulfate. After the ether had been removed, the deuteroThe ketone boiled at 105.7-106.3

Kinetics of Reactions in Solution

“…While the rate factor kHI kD = 10 is not too large to be explained by the usual means, Westheimer speculates that the reason may be a contribution to the reaction by a branching chain path involving C-H bond breaking, thus magnifying the normal isotope effect (171). Of two CrVI oxidation reactions shown to involve C-H bond breaking in the rate-determining step, Ropp & Hodnett (172) find a C14 isotope rate effect in one of the reactions.…”

Unsaturated carbenes formed by reaction of nitrosooxazolidinones with bases

“…Cr(V) + substrate -+ Cr(1V) + radical (1) Cr(IV) + Cr(V1) + 2Cr(V) (2) further experimental details, will be presented in a full paper. …”

Rate and product studies on the oxidative cleavage of some bicyclic alcohols with ceric ammonium nitrate

“…As with chromium oxidations (20)(21)(22)(23)(24)(25)(26)(27)(28)(29), eerie ammonium nitrate appears to oxidize a more crowded alcohol faster. This is readily seen in comparing epimeric pairs.…”